This invention relates to fluid flow meters of the constant differential pressure type wherein a movable element is provided which, in automatic response to changes in fluid flow rate, effects corresponding changes in cross-sectional area of the minimum flow passage thereof. The flow meter of my invention is especially adapted for accurately measuring gas flows over a wide range of rates while maintaining a highly linear relationship which can be extended readily to a cooperative electrically transduced indicator system of closely matched linear responsiveness, if desired.
Over the years considerable effort has been directed toward improving variable area type flow meters to make them more stable and reliable and more nearly linear and accurate over a wide range of flow rates. One of the most common forms of apparatus developed in this field is the so-called rotameter, a device in which a generally roundish float element is lifted inside of an upwardly expanding tapered tube to heights directly dependent upon the fluid flow rates therethrough. Due to their simplicity and responsiveness, rotameters are very popular but are too fragile and erratic for many applications due to such problems as float bounce, mechanical resonance and uncertain or variable alignments.
Most of the more rugged variable area flow meters developed to date have movable elements which are either inherently stiff or separately spring biased, or else encumbered with sufficient frictional effects to dampen their responsiveness and prevent true linearity of the position vs. flow rate relationship. Examples of this type of flow meter in which the movable element is deliberately counterbiased with a back up spring are disclosed in U.S. Pat. No. 3,234,790 to Ekstrom and U.S. Pat. No. 1,416,220 to Long et al. On the other hand, Ongaro in U.S. Pat. No. 3,218,853 has advocated the use of a primarily gravity opposed movable element in such flow meters provided said movable element is vertically aligned. However, said patentee also points out the extreme difficulty of eliminating mechanical interferences and frictional resistances in any such device which is capable of durable service over a reasonable range of operating conditions.
The primary object of this present invention is to provide an essentially constant differential pressure, variable area flow metering device in which the movable element is truly gravity opposed and mechanical friction is effectively eliminated thereby achieving a very smooth linear response between fluid flow rate and displacement of the movable element. It is further object of my invention to provide such a device which is durable and can be used continuously and/or repeatedly to deliver a smoothly linear and unchanging mechanical response to flow rates varying over a wide range (e.g. 10/1 or higher ratios). It is also one of my objects to provide such an apparatus which is compact and simple to use and operate.
A secondary object of this invention is to provide a directly coupled electric transducer which has a smoothly linear output closely matched to the mechanical displacement of the movable element in the variable area metering device over its full operating stroke. Finally, it is a special object of my invention to provide throughout my apparatus the capability of using corrosion resistant materials of construction so that it can be used to measure the flow rates of such aggressive gases as chlorine, oxides of nitrogen or sulfur, anhydrous ammonia etc.